1. Field of the Invention
This invention relates to an image formation system for forming multicolor images.
2. Description of the Related Art
FIG. 16 is a schematic block diagram to show an example of a conventional image formation system. In the figure, numeral 1 is an image reader, numeral 2 is an image writer, numeral 3 is a photosensitive body, numeral 4 is a developing machine, numeral 5 is a transfer device, numeral 6 is a cleaner, numeral 7 is an intermediate transfer belt, numeral 8 is a drive roll, numeral 9 is a cleaner, numeral 10 is paper, numeral 11 is a fuser, numeral 12 is a transfer roll, and numeral 13 is a control section. In the example, four image formation sections each consisting of the image writer 2, the photosensitive body 3, the developing machine 4, the transfer device 5, and the cleaner 6 are provided for forming images of different colors on the intermediate transfer belt 7, for example, forming color images of Y (yellow), M (magenta), C (cyan), and K (black). The control section 13 controls the components for forming images as described below.
A color image read through the image reader 1 or supplied from an external system is supplied to the image formation sections corresponding to the colors. In each image formation section, the image writer 2 forms a latent image on the photosensitive body 3, the developing machine deposits the corresponding color toner on the photosensitive body 3 for development, and the transfer device 5 transfers the image to the intermediate transfer belt 7. Unnecessary toner is collected in the cleaner 6.
The color images formed in the image formation sections are thus formed on the intermediate transfer belt 7 in overlapped relation. The color images transferred to the intermediate transfer belt 7 are transferred to paper 10 by means of the transfer roll 12 and fused on the paper by the fuser 11. A final color image is thus formed on the paper 10.
Unnecessary toner on the intermediate transfer belt 7 is collected in the cleaner 9.
The intermediate transfer belt 7 is turned by the drive roll 8. Thus, if the speed of the intermediate transfer belt 7 varies, the formation positions of color images become different, causing a color shift, inconsistencies in density, etc. The transport speed of the intermediate transfer belt 7 is represented by product of angular velocity of the drive roll 8 and distance L between the rotation center of the drive roll 8 and the intermediate transfer belt 7. Possible causes of a color shift and inconsistencies in density are variation in the angular velocity caused by eccentricity of the drive roll 8 and variation in the distance L between the rotation center of the drive roll 8 and the intermediate transfer belt 7.
FIG. 17 is an illustration of speed variation of the intermediate transfer belt 7, wherein the radius of the drive roll 8 is r, the thickness of the intermediate transfer belt 7 is D0, and the transport speed of the intermediate transfer belt 7 is Vb. The distance L between the rotation center of the drive roll 8 and the intermediate transfer belt 7 is the radius r of the drive roll 8 plus a half of the thickness D0 of the intermediate transfer belt 7 (r+D0/2). As described above, the transport speed Vb of the intermediate transfer belt 7 is EQU Vb=L.multidot..omega.=(r+D0/2).multidot..omega.
If the drive roll 8 is eccentric by .delta.r, EQU Vb=L.multidot..omega.=(r+.delta.r+D0/2).multidot..omega.
Therefore, transport speed difference .DELTA.Vb is EQU .DELTA.Vb=.delta.r.multidot..omega.
Hitherto, eccentricity of the drive roll 8 has been thought of as causes of a color shift and inconsistencies in density. For example, image formation section spacing is set to the move distance of intermediate transfer belt 7 as much as n revolutions of drive roll 8 and all color images are formed in synchronization with the speed variation caused by eccentricity of the drive roll 8 for preventing a color shift, inconsistencies in density, etc., as described in Japanese Patent Laid-Open No. Hei 4-172376. In fact, however, if images are formed in synchronization with the eccentricity of the drive roll 8, a color shift and inconsistencies in density occur. For such a color shift and inconsistencies in density, sinusoidal variation occurs over one cycle of the intermediate transfer belt 7, for example. Such variation is caused by unevenness in thickness of the intermediate transfer belt 7. For example, if the intermediate transfer belt 7 is made of a seamless belt, the unevenness in thickness occurs due to the belt manufacturing method.
Assuming that the change amount in the thickness of the intermediate transfer belt 7 is .delta.D, EQU Vb=L.multidot..omega.=(r+(D0+.delta.D)/2).multidot..omega. EQU .DELTA.Vb=(.delta.D/2).multidot..omega.
from the above-described expressions, and transport speed difference .DELTA.Vb occurs. Thus, if the intermediate transfer belt 7 contains unevenness in thickness, speed variation also occurs. This speed variation becomes longer-term variation than the speed variation caused by the eccentricity of the drive roll 8.
FIGS. 18A and 18B are illustrations of a color shift caused by speed variation of the intermediate transfer belt 7 in the conventional image formation system. The graphs shown in FIGS. 18A and 18B show position shift amounts from the normal position within the time of one cycle of the intermediate transfer belt 7 and, for example, indicate that the transport speed becomes fast in the rising portion and slows down in the falling portion. As shown in FIG. 18A, color images are formed in sequence with transport of the intermediate transfer belt 7. First, a K (black) image is formed and the K (black) image formation portion is transported to the subsequent Y (yellow) image formation section for superposing a Y (yellow) image on the K (black) image. Likewise, M (magenta) and C (cyan) images are also formed in time sequence indicated by the arrows in FIG. 18A and are superposed.
FIG. 18B shows position shift amounts in the formation ranges of the color images, wherein the K (black) position shift amount is indicated by a solid line, the Y (yellow) position shift amount is indicated by a dotted line, the M (magenta) position shift amount is indicated by a dashed line, and the C (cyan) position shift amount is indicated by a dot-dash line. For example, if image position shift when the M (magenta) image is formed is assumed to be plus above 0 and minus below 0, it belongs to the minus side almost throughout the zone. However, when the C (cyan) image is formed, the transport speed of the intermediate transfer belt 7 becomes fast and the position shift belongs to the minus side in the beginning portion of the image and the plus side in the last portion of the image. Since the position shift amount varies from one color to another as shown in FIG. 18B, a color shift occurs.
FIG. 19 is a schematic block diagram to show another example of a conventional electrophotographic printer. Parts similar to those previously described with reference to FIG. 16 are denoted by the same reference numerals in FIG. 19. In the example, only one image formation section is provided and while color of toner deposited on a photosensitive body 3 by a developing machine 4 is changed, color images are formed. For example, first a K (black) image is formed on the photosensitive body 3, is transferred to an intermediate transfer belt 7, and is formed on paper 10. Then, the toner of the developing machine 4 is changed and a Y (yellow) image is formed. Subsequently, M (magenta) and C (cyan) images are formed in a similar manner.
FIGS. 20A and 20B are illustrations of a color shift caused by speed variation of intermediate transfer belt 7 in another example of a conventional electrophotographic printer. In the configuration, an image is formed on the intermediate transfer belt 7 for each color. At this time, generally the intermediate transfer belt 7 is designed to make one revolution for each color. Thus, if the speed of the intermediate transfer belt 7 varies as shown in FIG. 20A, the position shift amounts of the colors are synchronized with each other as shown in FIG. 20B; although partial shrinkage or extension exists on the actually formed image, a color shift, inconsistencies in density, or the like does not occur. In the example, the length of the intermediate transfer belt 7 corresponds to the image length, thus the time axis of the graph shown in FIG. 20A differs from the time axis scale shown in FIG. 18A.
In such a configuration, the length of the intermediate transfer belt 7 must be limited to the maximum length of an image or 1/n of the maximum length or image formation must always be started at a predetermined position. If the intermediate transfer belt 7 is set to any desired length and image formation is started at any desired position, a color shift occurs as in the configuration shown in FIG. 16.
In the examples, an image is once formed on the intermediate transfer belt 7 and is transferred to paper. If paper is transported on a belt and an image is transferred from a photosensitive body directly to the paper, a color shift and inconsistencies in density are also caused by unevenness in the belt transport speed.
Some techniques are designed for sensing speed variation and position variation of the intermediate transfer belt 7 and the transport belt and correcting the image formation position. For example, sense means are described in Japanese Patent Laid-Open Nos. Hei 4-172376, 4-234064, etc., wherein an encoder is attached to a roll shaft driven by a belt and the belt speed is sensed from the angular velocity.
FIGS. 21 and 22 are illustrations of examples of conventional speed variation sense means. In the figures, numeral 41 is a belt, numeral 42 is an encoding roll, numeral 43 is a bearing, numeral 44 is an encoder, and numeral 45 is a pinch roll. The encoding roll 42 is rotated with transport of the belt 41 and the encoder 44 is rotated via the bearing 43, then the rotation speed is detected.
However, in the configuration in which the belt 41 is placed on the encoding roll 42 as shown in FIG. 21, the portion of the encoding roll 42 is also affected by unevenness in thickness of the belt 41, eccentricity of the encoding roll 42 itself, etc., and speed variation caused by the unevenness in thickness of the belt 41 cannot accurately be measured. In the system in which the pinch roll 45 presses the belt 41 against the encoding roll 42 and the encoding roll 42 is rotated as shown in FIG. 22, the belt 41, which is sandwiched between the two rolls, is easily subjected to damage and reliability is not ensured.
FIG. 23 is an illustration of another example of conventional speed variation sense means. In the figure, numeral 46 is a mark and numeral 47 is a sensor. For example, as described in Japanese Patent Laid-Open No. Hei 6-130871, another speed variation sense method is also available wherein marks 46 are previously printed on a belt 41 and are sensed by sensor 47, whereby the speed of the belt 41 is sensed. However, it is difficult to print the marks 46 accurately, thus the method involves a problem in precision.
On the other hand, so-called registration control technique for registering the start positions, etc., of images formed by image formation sections is known, for example, as described in Japanese Patent Laid-Open No. Hei 6-253151. The registration technique described here is to form an image on a belt in each image formation section, detect the image by a sensor, and correct an image position shift in each image formation section. In the conventional registration control technique, write position variation among the image formation sections is only corrected and a color shift and inconsistencies in density in images caused by belt speed variation as described above are not corrected.